Tuning Up the Hall Thruster

byPaul GilsteronFebruary 23, 2007

A nice upgrade to existing satellite engine technology comes out of Georgia Tech, where researchers have developed a design that allows the engine to optimize available power, much like the transmission of a car. Thus the engine can burn at full throttle in ‘first gear,’ maximizing acceleration, while dropping into a much more economical gear for long-term space operations. “You can really tailor the exhaust velocity to what you need from the ground,” says team leader Mitchell Walker.

The engine at work here is known as a Hall effect thruster, a plasma-based propulsion system that operates with xenon, a gas that is injected into a discharge chamber where its atoms become ionized. The electrons that are stripped from the outer shell are trapped in a magnetic field, while the heavier xenon ions are accelerated out into space by an electric field. What Georgia Tech has introduced is better control over the exhaust stream through an enhanced electric and magnetic field design.

Image: Georgia Tech’s enhanced engine uses a novel electric and magnetic field design that helps better control the exhaust particles. Ground control units can then exercise this control remotely to conserve fuel. Credit: Georgia Institute of Technology.

A key factor is specific impulse (ISP), which measures how much thrust is produced per unit of fuel in each second of an engine’s burn. Looked at another way, ISP is a measure of how many seconds one pound of propellant can produce one pound of thrust. As specific impulse (stated in seconds) rises, it takes less fuel to produce a given amount of thrust, and the amount of payload compared to propellant can also rise.

In a telephone interview, Walker told me that the Georgia Tech work is not about creating a new engine but pushing existing engines into regimes in which they normally don’t function well:

We took an engine that normally runs at 2500 seconds and we backed it down to 1000 seconds. In other words, we traded exhaust velocity for more thrust. When we did that the engine efficiency dropped from 65 percent down to about 25 percent — the engine did not like to run there. What we’ve been able to do is to focus ions that would otherwise crash into the chamber wall to create huge efficiency losses. That drives the efficiency of the engine back up while running at 1000 seconds.

All of which is good news for various space missions, since the design — modified from a donated Pratt & Whitney satellite engine — can reduce onboard fuel needs by 40 percent, thus freeing up space for payload. An already efficient engine thereby becomes more stingy still with its fuel using proven technologies. Remember that Deep Space 1, launched in 1998, tested out ion propulsion using xenon, producing only one-fiftieth a pound of thrust at full throttle, but with high specific impulse.

For that matter, the European Space Agency’s SMART-1 lunar mission used solar-electric methods, with the electricity generated from its solar panels being used to accelerate xenon ions. Today’s low thrust ion engines are so efficient that they can run for months or years. Researchers at the Jet Propulsion Laboratory operated an NSTAR thruster for a continuous 30,352 hours — these engines are workhorses — and both power and specific impulse are being improved in ion thruster designs like NEXT, the NASA Evolutionary Xenon Thruster.

A good backgrounder on ion propulsion from New Scientist is here, with details on NEXT. The classic book on the subject is Robert Jahn’s The Physics of Electric Propulsion (McGraw-Hill, 1968). A 2006 paperback edition from Dover brings this core text back onto bookstore shelves.

Subject the Proton drive:
A proton drive is an ion engine that accelerates hydrogen ions, or protons
into an exchaust jet . Synchrotons, cyclotrons, and linear acclerators
all can accelerate beams of protons to near the velocity of light .
The protons can then be neutralized and expelled to generate thrust .
Atomic or fusion reactors are onboard power sources for this , and
there are maser electric, and laser electric external power sources for
proton drive rockets and ramjet engines . All you have to do is provide
enough energy to run it from an internal, or external source of power.

There have not been many comments on this thread but I think it is a cool thread nonetheless.

It occurred to me that, if some how, a material with a permanent electric field could be used to fabricate an a accelleration chamber composed of electrics, the electric field analogue of a magnet (electrics are used in commercial utility power grid generators), one might be able to produce a chamber that would accellerate electrons that were dissassociated from positrons down a voltage gradient thusting chamber while at the same time, the positrons could be accelerated down a voltage gradient chamber having a reverse polarity with respect to the electron chamber. The voltage difference between the two electric field chambers could be maintained in such a way that the electric fields do not cancel each other out by some sort of electric field shielding barrier such as a dielectric material or what not.

If a large enough potential difference could be maintained along the accellerating length of the chambers, a extraordinarilly high rest mass specific Isp might be obtained by storing vast quantities of the material positronium within the ship and then some how dissassociating it without causing it to explode in a huge matter antimatter explosion.

What’s more, perhaps the exhuast beams of positrons and electrons could be made to interact behind a gamma radiation sail or pusher plate to propell the space craft forward for additional energy effeciency.

Now if we can just find a particle or particles with a charge to mass ratio greater than that of an electron, preferably much greater, we would be in great luck. Perhaps the proposed RHIC upgrade or the LHC of CERN or perhaps a proposed 30 kilometer long TeV range electron linac might yeild some results here. If not, perhaps more powerful machines could.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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